Crucial impact of exchange between layers on temperature programmed desorption

Desorption of molecules from surfaces constitutes an elementary process that is fundamental in both natural and application-oriented fields, including dewetting, weathering and catalysis. A powerful method to investigate desorption processes is temperature-programmed desorption (TPD), which offers the potential to provide mechanistic insights into the desorption kinetics. Using TPD, the desorption order, the energy barrier as well as the entropy change upon desorption can be accessed. In the past, several analysis methods have been developed for TPD data. These methods have in common that they rely on the Polanyi-Wigner equation, which requires proposing a desorption mechanism with a single (or at least dominating) desorption path. For real systems, however, several coupled desorption paths can be easily envisioned, which cannot be disentangled. Here, we analyse the influence of exchange between the first and the second adsorbate layer on the desorption process. We present a kinetic model, in which molecules can desorb directly from the first layer or change into the second layer and desorb from there. Interestingly, considering this additional desorption pathway alters the desorption spectrum considerably, even if the transient second-layer occupation remains as small as 4 × 10^-6 monolayers. We show that the impact of this layer exchange can be described by a modified Polanyi-Wigner equation. Our study demonstrates that layer exchange can crucially impact the TPD data.